The heater winding which is one the same transformer as the HT winding will typically produce a noisy voltage, due to the HT rectifier. A dedicated heater transformer should be as clean as the mains. If you're regulating, though, then this matters little.

However, regulating the EL34's supply will require some serious capacitance!

If it were me (I'm cheap!) I would supply the EL34s from the stock heater winding, unregulated, since the EL34s are so insensitive that they should be practically hum free anyway. I would then use the separate heater transformer to supply regulated DC to all the preamp valves.

I agree, since the EL34s are in PP topology, there is no need to regulate them. So I will add a 6A plus, winding to the toroid and use it to power all 4 EL34 heaters, and use the 6.3V 4A winding that comes on the toroid already to provide regulated heater supply to all the drivers.

I really don't want to add a separate heater transformer, that is part of the reason to regulate the driver heaters.

I have gotten no response to this in the power supply forum, so I'll try here. You all are super helpful.

I have a 400VA toroid power transformer with 400VCT, 5V 4A and 6.3V 4A secondaries.

I am planning to use it in an EL34 PP amp, so for the heaters I need 1.5A per EL 34 and 0.3A per driver (12AT7).

Use this transformer for the regulated 6.3V supply. Then use a larger one fo rht unregulated supply to feed the power tubes. This way your primary 400VA transformer will run cooler and last maybe forever. A 6.3V 6A heater transformers is cheap,run the power tubes off that.

Use this transformer for the regulated 6.3V supply. Then use a larger one fo rht unregulated supply to feed the power tubes. This way your primary 400VA transformer will run cooler and last maybe forever. A 6.3V 6A heater transformers is cheap,run the power tubes off that.

I understand, but even if I wrap a new winding around my current toroid core for the 6.3 6A supply, I will not exceed 200VA total and the toroid is supposedly underrated at 400VA.

Seems like I am not even pushing it, so why buy and have to find mounting space for another transformer?

Keep in mind that te VA rating to actual W you take out of the transformer, depends on how you do your rectifying and filtering. VA will be equal to W only for a purely resistive load, which a rectifier most definitely is NOT. Tube heaters, however, are, but only if you supply them with AC directly. Thier resistance increases with temperature so the initial current dray will be quite a bit higher but it will only last a few seconds.
Rectifiers, in particular solid state ones, are far more tricky - because of capacitor filtering, the peak currents through them are many times that of an equivalent resistive load taking equal power, because they only operate during a small part of the AC waveform, around the peak. Because of this it's the peak current and not the power needed that will determine the transformer VA rating, and it will be higher (sometimes a lot higher) than the actual W you take out of it. If this is violated, core saturation occurs, and even before that, you have to keep in mind that the transformer effect works not only between primary and secondary windings, but between every winding in all combinations.
One typical problem is rectifying heater supplies and filtering with large capacitors, where the ehater winding is one of several on a much larger core. The heater rectifier will be subjected to much higher current pulses than expected because the large core makes it possible to, in theory, take all of the core limited VA through one winding (because the primary is made for the total VA rating). To make things worse, these current pulses leak into the other secondaries in form of voltage drops, especially problematic if there are other rectifiers on other windings. The whole problem is made worse by the requirement for very large caps to filter the relatively low heater voltage at relatively high currents. In case you plan going this way, it is a good idea (though heat is generated...) to add series resistors between the rectifier and filter cap, to limit the maximum peak current. If you are going to use tube rectifiers for B+, the problem islessened by the fact that they have a rather high internal resistance which acts as a low pass filter together with the filter cap, removing most of the HF hash the heater rectifier will generate, from the B+ line.
Another consideration regarding heater windings is adhering to the Vhk specs for all the tubes in your amp, also, for heater 'ground lift' if required. Such things may make separate heater windings for parts or even only one stage of the amp a necessity (typically SRPP stages, and in some cases concertina phase splitters and voltage followers may require this).

The transformer is 400VA and will run the B+ around 400V and require a B+ current of no more than 0.3A
Tube rectified.
Rectifier heater winding on main toroid, 5V @ 1.9A
Driver heater winding on main toroid (have to use this one as it is not rated high enough for the 4 power tubes), 6.3V 0.6A (rated 4A). This is the only regulated heater winding.

Then I need an additional 6.3V @ 6A for the power tubes. this will not be regulated and will be AC, since the topology is PP (that should be fine, right?).

I would prefer to add this winding the the existing toroid, rather than purchase a dedicated heater transformer. Now, given your post, ilimzn, i have to ask your advice if you think the above plan would allow that without trouble. the rectifier is a GZ34, if that matters. Thanks!

I understand, but even if I wrap a new winding around my current toroid core for the 6.3 6A supply, I will not exceed 200VA total and the toroid is supposedly underrated at 400VA.

Seems like I am not even pushing it, so why buy and have to find mounting space for another transformer?

First off as has been pointed out VA equals watts only for DC. or AC with a pure resistive load. Under any other condition impedance is a complex number.

Also you don't want to run the B+ transformer near it's rated limit. You depend of the over rating for load regulation. Although you are right your transformer is larger than you need

When you design the regulated 6.3 supply the 6.3 VAC will be nearly 9 VDC and then your regulator will convert some of this into heat as it knocks the voltage back down to 6.3. As a rule of thumb, figure that a regulated supply will pull about 1.5 x more current from the transformer.